X.-G. Chen

Superhydrophobic Aluminum Alloy Surfaces by a Novel One-Step Process

A simple one-step process has been developed to render aluminum alloy surfaces superhydrophobic by immersing the aluminum alloy substrates in a solution containing NaOH and fluoroalkyl-silane (FAS-17) molecules. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and water contact angle measurements have been performed to characterize the morphological features, chemical composition and superhydrophobicity of the surfaces. The resulting surfaces provided a water contact angle as high as ∼162° and a contact angle hysteresis as low as ∼4°. The study indicates that it is possible to fabricate superhydrophobic aluminum surfaces easily and effectively without involving the traditional two-step processes.

Chemical Nature of Superhydrophobic Aluminum Alloy Surfaces Produced via a One-Step Process Using Fluoroalkyl-Silane in a Base Medium

Various surface characterization techniques were used to study the modified surface chemistry of superhydrophobic aluminum alloy surfaces prepared by immersing the substrates in an aqueous solution containing sodium hydroxide and fluoroalkyl-silane (FAS-17) molecules. The creation of a rough micronanostructure on the treated surfaces was revealed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS) confirmed the presence of low surface energy functional groups of fluorinated carbon on the superhydrophobic surfaces. IRRAS also revealed the presence of a large number of OH groups on the hydrophilic surfaces. A possible bonding mechanism of the FAS-17 molecules with the aluminum alloy surfaces has been suggested based on the IRRAS and XPS studies. The resulting surfaces demonstrated water contact angles as high as ~166° and contact angle hystereses as low as ~4.5°. A correlation between the contact angle, rms roughnesses, and the chemical nature of the surface has been elucidated.

Effect of Mn, Si, and Cooling Rate on the Formation of Iron-Rich Intermetallics in 206 Al-Cu Cast Alloys

The solidification structures of commercial 206 Al-Cu cast alloys with 0.15xa0pct Fe have been studied using thermal analysis (TA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD). The EBSD results have shown that there are two iron-rich intermetallics: Chinese script α-Fe and platelet-like β-Fe. The addition of either Mn or Si has helped to promote the formation of α-Fe and hinder the precipitate of β-Fe. The combined addition of both Mn and Si is even more effective than the individual addition of either Mn or Si. The full solidification sequence of the 206 cast alloy has been established. The volume percent and formation temperature increase for α-Fe but decrease for β-Fe with increasing cooling rate. The platelet β-Fe can be effectively suppressed in 206 cast alloys by controlling the alloy chemistry and cooling rate. A casting process map is proposed to correlate the Mn and Si contents with cooling rates for the 206 cast alloys.

A New Iron-Rich Intermetallic-Al m Fe Phase in Al-4.6Cu-0.5Fe Cast Alloy

A new iron-rich intermetallic, AlmFe, is observed, for the first time, in the important A206 (Al-4.6Cu-0.5Fe) cast alloy and identified using electron backscattered diffraction (EBSD) and a transmission electron microscope (TEM). Results show that Chinese script AlmFe can precipitate as the dominant iron-rich intermetallic at high iron content (0.5xa0wtxa0pct) and low cooling rate (0.2xa0K/s). EBSD and TEM results confirmed that AlmFe has body-centered-tetragonal structure with an average m of approximately 4.2.

Fabrication of Superhydrophobic Surfaces on Aluminum Alloy Via Electrodeposition of Copper Followed by Electrochemical Modification

Superhydrophobic aluminum surfaces have been prepared by means of electrodeposition of copper on aluminum surfaces, followed by electrochemical modification using stearic acid organic molecules. Scanning electron microscopy (SEM) images show that the electrodeposited copper films follow “island growth mode” in the form of microdots and their number densities increase with the rise of the negative deposition potentials. At an electrodeposition potential of −0.2 V the number density of the copper microdots are found to be 4.5×104 cm−2 that are increased to 2.9×105 cm−2 at a potential of −0.8 V. Systematically, the distances between the microdots are found to be reduced from 26.6 μm to 11.03 μm with the increase of negative electrochemical potential from −0.2V to −0.8V. X-ray diffraction (XRD) analyses have confirmed the formation of copper stearate on the stearic acid modified copper films. The roughness of the stearic acid modified electrodeposited copper films is found to increase with the increase in the density of the copper microdots. A critical copper deposition potential of −0.6V in conjunction with the stearic acid modification provides a surface roughness of 6.2 μm with a water contact angle of 157°, resulting in superhydrophobic properties on the aluminum substrates.

Formation and Phase Selection of Iron-Rich Intermetallics in Al-4.6Cu-0.5Fe Cast Alloys

The solidification structures of Al-Cu 206 cast alloys at a high iron level of 0.5xa0pct were systematically studied by means of differential scanning calorimetry, electron backscattered diffraction, and transmission electron microscopy. The full solidification sequences of the 206 cast alloys at 0.5xa0pct Fe were established. The influences of both alloy composition (i.e., Si and Mn contents) and cooling rate on the formation and phase selection of the iron-rich intermetallics have been systematically explored. At a cooling rate of 12xa0K/min, it was found that one of the three iron-rich phases, i.e., Chinese script Alm(FeMn) and α-Fe, or platelet Al3(FeMn), may precipitate as the dominant iron-rich intermetallic, depending on Si and Mn contents. However, the dominant Chinese script iron-rich intermetallics, Alm(FeMn) and/or α-Fe, can be fully obtained for the 206 Al-Cu cast alloys at 0.5xa0pct Fe above a threshold cooling rate that can easily be obtained in normal industrial casting conditions, indicating that there is a significant potential of designing and developing new 206 Al-Cu cast alloys with a high tolerant iron content.

Tensile Properties of Al-Cu 206 Cast Alloys with Various Iron Contents

The Al-Cu 206 cast alloys with varying alloy compositions (i.e., different levels of Fe, Mn, and Si) were investigated to evaluate the effect of the iron-rich intermetallics on the tensile properties. It is found that the tensile strength decreases with increasing iron content, but its overall loss is less than 10xa0pct over the range of 0.15 to 0.5xa0pct Fe at 0.3xa0pct Mn and 0.3xa0pct Si. At similar iron contents, the tensile properties of the alloys with dominant Chinese script iron-rich intermetallics are generally higher than those with the dominant platelet phase. In the solution and artificial overaging condition (T7), the tensile strength of the 206 cast alloys with more than 0.15xa0pct Fe is satisfactory, but the elongation does not sufficiently meet the minimum requirement of ductility (>7xa0pct) for critical automotive applications. However, it was found that both the required ductility and tensile strength can be reached at high Fe levels of 0.3 to 0.5xa0pct for the alloys with well-controlled alloy chemistry and microstructure in the solution and natural aging condition (T4), reinforcing the motivation for developing recyclable high-iron Al-Cu 206 cast alloys.

Evolution of intermetallics, dispersoids and elevated-temperature properties at various Fe contents in Al-Mn-Mg 3004 alloys

Nowadays, great interests are rising on aluminum alloys for the applications at elevated temperature, driven by the automotive and aerospace industries requiring high strength, light weight, and low-cost engineering materials. As one of the most promising candidates, Al-Mn-Mg 3004 alloys have been found to possess considerably high mechanical properties and creep resistance at elevated temperature resulted from the precipitation of a large number of thermally stable dispersoids during heat treatment. In present work, the effect of Fe contents on the evolution of microstructure as well as high-temperature properties of 3004 alloys has been investigated. Results show that the dominant intermetallic changes from α-Al(MnFe)Si at 0.1 wtxa0pct Fe to Al6(MnFe) at both 0.3 and 0.6xa0wtxa0pct Fe. In the Fe range of 0.1–0.6xa0wtxa0pct studied, a significant improvement on mechanical properties at elevated temperature has been observed due to the precipitation of dispersoids, and the best combination of yield strength and creep resistance at 573xa0K (300xa0°C) is obtained in the 0.3xa0wtxa0pct Fe alloy with the finest size and highest volume fraction of dispersoids. The superior properties obtained at 573xa0K (300xa0°C) make 3004 alloys more promising for high-temperature applications. The relationship between the Fe content and the dispersoid precipitation as well as the materials properties has been discussed.

Solid-State Transformation of Iron-Rich Intermetallic Phases in Al-Cu 206 Cast Alloys During Solution Heat Treatment

The solid-state transformation of the iron-rich intermetallic phases in Al-Cu 206 cast alloys during the solution heat treatment was studied by using Scanning Electron Microscope (SEM), Electron Back Scattered Diffraction (EBSD), Differential Scanning Calorimeter (DSC), and Transmission Electron Microscope (TEM). At a normal solution treatment temperature of 793xa0K (520xa0°C), no visible variation is observed for the β-Fe phase solidified from the Al alloy melt. With increasing soaking time, however, the Chinese script α-Fe becomes unstable and progressively transforms into platelet β-Fe, termed as solid-state-transformed (STed) β-Fe to distinguish it from the β-Fe directly solidified from the Al alloy melt. The STed β-Fe preferentially nucleates on the α-Fe and then grows from the α-Fe/Al interface into α-Fe and/or Al matrix with a much higher growth rate in the α-Fe. The incomplete solid-state transformation from α-Fe into STed β-Fe leads to the fragmentation of the α-Fe. The formation of the STed β-Fe with increasing size and volume fraction after longer soaking time can deteriorate the tensile properties.

Effect of magnesium on dispersoid strengthening of Al—Mn—Mg—Si (3xxx) alloys

Abstract The effects of magnesium addition on the dispersoid precipitation as well as mechanical properties of 3xxx alloys were investigated. The microstructures in as-cast and heat-treated conditions were evaluated by optical microscopy and transmission electron microscopy. The results reveal that Mg has a strong influence on the distribution and volume fraction of dispersoids during precipitation heat treatment. The microhardness and yield strength at ambient temperature increase with increasing Mg content. The solid solution and dispersoid strengthening mechanisms of materials after heat treatment are quantitatively analyzed. Dispersoid strengthening for the alloys is the predominant strengthening mechanism after precipitation heat treatment. An analytical model is introduced to predict the evolution of ambient-temperature yield strength.